BAGHDAD (Reuters) - At least four people were killed by bomb blasts in Iraq on Tuesday and mortars targeted a police training facility, security and medical sources said, wounding two officers.

There was no immediate claim of responsibility for the attacks, but Islamic State militants who seized swathes of northern and western Iraq last year regularly target security and civilian targets.

Iraq is also gripped by a sectarian conflict mostly between Shi'ite and Sunni Muslims that has been exacerbated by the rise of the ultra-hardline Sunni group, Islamic State.

Two civilians were killed by separate roadside bombs in the greater Baghdad area, police and medics said. One blast was in a predominately Shi'ite area north of the capital, and the other in a mixed neighborhood in the south.

Two pro-government Shi'ite militia fighters were killed when a suicide bomb in a truck exploded at a gathering of security forces near the northern city of Samarra, a military source said. Clashes broke out at the site of the bomb, which left seven police officers wounded.

Further north, two police officers at Camp Speicher, a former U.S. base outside the Sunni city of Tikrit, were wounded in a mortar attack while new recruits were doing training exercises, the military source said.

A U.S.-led coalition, which has been bombing Islamic State targets for more than a year, has helped contain the group, but some Iraqi politicians and powerful Shi'ite militias fighting alongside Iraq's military and police question U.S. resolve to defeat the Sunni insurgents.

U.S. Defense Secretary Ash Carter told a Senate hearing on Tuesday that U.S. forces aimed to intensify pressure on Ramadi, the capital of western Anbar province which Islamic State seized in May.

U.S. Marine General Joseph Dunford, chairman of the Joint Chiefs of Staff, told lawmakers he would consider recommending putting U.S. forces with Iraqi troops to fight Islamic State if that would improve the chances of defeating the militants.

U.S. and Kurdish special forces raided a compound in northern Iraq last week, rescuing 69 people detained by Islamic State in the most significant raid against the group since May. One U.S. serviceman was killed.